1. Field of the Invention
The present invention relates to a polymer electrolyte membrane for a fuel cell, and a fuel cell system including the same.
2. Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material such as methanol, ethanol, or natural gas. Such a fuel cell is a clean power source. It includes a stack composed of unit cells and can produce various ranges of power output. Since it has four to ten times higher energy density than a small lithium battery, it has become highlighted as a small portable power source.
Typical examples of fuel cells are polymer electrolyte membrane fuel cells (PEMFC) and direct oxidation fuel cells (DOFC). A direct oxidation fuel cell which uses methanol as a fuel is called a direct methanol fuel cell (DMFC).
A polymer electrolyte fuel cell provides advantages which include high energy density, but it also has problems in the need to carefully handle hydrogen gas, and the requirement of accessory facilities such as a fuel reforming processor for reforming methane, methanol, natural gas, or the like, in order to produce hydrogen as the fuel gas.
In contrast, a direct oxidation fuel cell has lower energy density than that of a polymer electrolyte fuel cell, but provides advantages which include easier fuel handling, the ability to operate at room temperature due to its low operation temperature, and the elimination of the need for additional fuel reforming processors.
In the above-mentioned fuel cell system, the stack that generates electricity substantially includes several unit cells stacked adjacent to one another. Each unit cell is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly is composed of an anode (also referred to as a “fuel electrode” or an “oxidation electrode”) and a cathode (also referred to as an “air electrode” or a “reduction electrode”) that are separated by a polymer electrolyte membrane.
A fuel is supplied to an anode and adsorbed on an anode catalyst where the fuel is oxidized to produce protons and electrons. The electrons are transferred into a cathode via an external circuit, and the protons are transferred into the cathode through the polymer electrolyte membrane. In addition, an oxidant is supplied to the cathode where it reacts with protons and electrons on a cathode catalyst to produce electricity along with water.
The polymer electrolyte membrane electrically insulates an anode and a cathode, transfers protons from the anode to the cathode during cell operation, and also separates gas and liquid reactants. Accordingly, a polymer electrolyte membrane should have excellent electrochemical stability, low ohmic loss at high current density, good separation properties between gas and liquid reactants during cell operation, and good mechanical properties and dimensional stability for stack fabrication.
As for the polymer electrolyte membrane, a perfluorinated sulfonic acid-based cation exchange resin (trade name: NAFION™) that E.I. du Pont de Nemours, Inc. developed in 1968 has been widely used. NAFION™ has hydrophobic polytetrafluoroethylene as a main chain and a functional group including a hydrophilic sulfone group at its side chain.
A NAFION™ polymer electrolyte membrane which is commercially available has many more advantages than a hydrocarbon-based polymer electrolyte membrane in terms of oxygen solubility, electrochemical stability, durability and the like. Since a NAFION™ polymer electrolyte membrane is proton-conductive when about 20% of the polymer weight therein becomes hydrated (i.e., a sulfone group included in a pendant group is hydrolyzed into sulfonic acid), a reaction gas used in a fuel cell should be saturated by water to hydrate the electrode membrane. However, the water gradually dries when operated at higher than the 100° C. boiling point of water, and accordingly the resistance of the polymer electrolyte membrane increases, deteriorating cell performance.
In addition, a NAFION™ polymer electrolyte membrane, which is commonly 50 to 175 μm thick, is prepared in a form of film by extrusion or solvent casting of a melted polymer. The NAFION™ polymer electrolyte membrane can be increased or decreased in thickness to improve dimensional stability and mechanical properties of a fuel cell. However, if it is increased in thickness, the conductivity of the polymer electrolyte membrane decreases, while if decreased in thickness, the mechanical properties are deteriorated.
In a methanol fuel cell, the polymer electrolyte membrane may allow liquid methanol fuel to pass through without reaction during the cell operation (referred to as methanol crossover), deteriorating cell performance as well as causing fuel loss, because the methanol is oxidized at the cathode.
A NAFION™ polymer electrolyte membrane also includes a hydrophobic area and a micro-phase hydrophilic cluster depending on temperature and a hydration degree. The hydrophobic area and micro-phase hydrophilic cluster repeatedly swell and shrink during operation and accordingly, induce membrane thickness and volume variation of about 15 to 20%. Therefore, in the case of a thin membrane, dimensional stability and interface properties between the electrode and electrolyte may be deteriorated.